1. Field
This disclosure relates to non-volatile memory.
2. Description of the Related Art
Semiconductor memory has become increasingly popular for use in various electronic devices. For example, non-volatile semiconductor memory is used in cellular telephones, digital cameras, personal digital assistants, mobile computing devices, non-mobile computing devices and other devices. Electrically Erasable Programmable Read Only Memory (EEPROM) and flash memory are among the most popular non-volatile semiconductor memories. With flash memory, also a type of EEPROM, the contents of the whole memory array, or of a portion of the memory, can be erased in one step, in contrast to the traditional, full-featured EEPROM.
Both traditional EEPROM and flash memory utilize a floating gate (FG) that is positioned above and insulated from a channel region in a semiconductor substrate. The floating gate is positioned between the source and drain regions. A control gate (CG) is provided over and insulated from the floating gate. The threshold voltage (VTH) of the transistor thus formed is controlled by the amount of charge that is retained on the floating gate. That is, the minimum amount of voltage that must be applied to the control gate before the transistor is turned on to permit conduction between its source and drain is controlled by the level of charge on the floating gate.
Some EEPROM and flash memory devices have a floating gate that is used to store two ranges of charges and, therefore, the memory element can be programmed/erased between two states, e.g., an erased state and a programmed state. Such a flash memory device is sometimes referred to as a binary flash memory device because each memory element can store one bit of data.
A multi-state (also called multi-level) flash memory device is implemented by identifying multiple distinct allowed/valid programmed threshold voltage ranges. Each distinct threshold voltage range corresponds to a predetermined value for the set of data bits encoded in the memory device. For example, each memory element can store two bits of data when the element can be placed in one of four discrete charge bands corresponding to four distinct threshold voltage ranges.
Typically, a read compare voltage VCGRV is applied to the control gate of a selected memory cell during a read operation. In some architectures, a read pass voltage (e.g., VREAD) is applied to control gates of unselected memory cells while the read compare voltage VCGRV is applied to the selected memory cell. The read pass voltage may be an appropriate magnitude to cause unselected memory cells to conduct.
Read disturb refers to undesirably altering the charge on a floating gate during a read operation. Altering the charge on the floating gate, alters the threshold voltage. With each read, the amount of disturb may be extremely small. However, after many reads, the effects may add up such that the threshold voltages are significantly altered. Note that in some cases, it may be unselected memory cells that suffer from read disturb.